1. Field of the Invention
The embodiments discussed herein pertain to a semiconductor device.
2. Description of the Related Art
As an insulated gate bipolar transistor (IGBT) based on silicon semiconductor, an IGBT that increases collector-emitter current density Jce (current density between collector and emitter) and reduces internal loss by enhancing a conductivity modulation effect in a drift layer has been developed.
Examples of such an IGBT is a high conductivity IGBT (HiGT) or a carrier stored trench-gate bipolar transistor that improves the carrier storage effect in an n− drift layer by inserting between a p-body region and the n− drift layer an n semiconductor layer (hereinafter “current density enhancement layer” (CEL)) in which the impurity concentration is higher than in the n− drift layer (see for example, see Japanese Patent Nos. 3288218, 4688901, 3395520, Japanese Laid-open Patent Publication No. 10-178174, and Oyama, K., et al, “Advanced HiGT with Low-injection Punch-through (LiPT) structure”, Proceedings of 2004 International Symposium on Power Semiconductor Devices & ICs, May 2004, pp. 111-114).
Recently, semiconductor materials (wide bandgap semiconductor materials) which have wider bandgaps than silicone such as silicone carbide (SiC) semiconductor are attracting attention. An IGBT with silicone carbide semiconductor (hereinafter “SiC-IGBT”) demonstrates superior performance such as low ON-state resistance, operability under high temperature, and large electric field intensity for electrical breakdown in comparison with an IGBT with silicone semiconductor. It has been proposed that the CEL is added into the SiC-IGBT to increase the collector-emitter current density Jce (see Japanese Laid-open Patent Publication No. 2008-211178 and Woongje Sung, et al, “Design and investigation of frequency capability of 15 kV 4H—SiC IGBT”, International Symposium on Power Semiconductor Devices & ICs 2009, July 2009, pp. 271-274). FIG. 6 is a sectional view of a semiconductor device for explaining a structure described in Japanese Laid-open Patent Publication No. 2008-211178 and Woongje Sung, et al, “Design and investigation of frequency capability of 15 kV 4H—SiC IGBT” as an example.
However, according to the above related art, the installment of the nCEL 75 of FIG. 6 can increase collector-emitter current density Jce but under the forward bias, a problem arises that breakdown voltage lowers because the intensity of the electric field near the pn junction of the p-body region 76 and the nCEL 75 increases. Especially, the electric field becomes strong at the corner (encircled portion) (hereinafter, the corner of the p-body region) below the p− channel 81 (encircled portion toward the gate oxide film) of the p body region 76. This kind of problem dominantly manifests itself when the impurity concentration in the nCEL 75 is raised or when the carrier storage effect is enhanced by increasing the thickness of the nCEL 75. When the electric field gets strong at the corner 82, the breakdown voltage lowers.
For example, Woongje Sung, et al, discloses a 15 kV SiC-IGBT where the n− drift layer 3 is chosen to be 150 μm thick and doped at 4.5×1014 cm−3, the nCEL 75 is chosen to be 3 μm thick and doped at 8.0×1015 cm−3, and the channel width (p− channel region 81) is 0.7 μm.
The SiC-IGBT of Woongje Sung, et al, “Design and investigation of frequency capability of 15 kV 4H—SiC IGBT” shows a property that when the breakdown voltage is 15 kV, the collector-emitter voltage Vce is 7.24 V and the collector-emitter current density Jce is 30 A/cm2, which is low. There is a trade-off relationship between the collector-emitter current density Jce and the breakdown voltage. Thus, it is difficult to improve both the collector-emitter current density Jce and the breakdown voltage.
The SiC-IGBT has a problem due to the characteristics of silicon carbide. Usually, the SiC-IGBT is formed on an epitaxial layer that is grown on a silicon carbide bulk substrate. In order to grow the epitaxial layer of 4H—SiC alone on the silicon carbide bulk substrate, an off-angle silicon carbide bulk substrate where the (0001) face (so-called “Si face”) is slanted some degrees with respect to a crystal axis is used.
When the off-angle silicon carbide bulk substrate is used, part of basal plane dislocations are converted to edge dislocations but the remaining basal plane dislocations are propagated into the epitaxial layer during epitaxial growth. If the impurity concentration in the epitaxial layer that grows on the silicon carbide is high, the conversion of the basal plane dislocations into the edge dislocations becomes difficult and many basal plane dislocations tend to be propagated into the epitaxial layer.
Since the conventional SiC-IGBT above forms the nCEL 75 by epitaxial growth, most of the basal plane dislocations propagating from the silicon carbide bulk substrate to the n++ buffer layer 2 and from the n++ buffer layer 2 to the n− drift layer 3 are propagated into the nCEL 75. The basal plane dislocations left in the epitaxial layer adversely affects a bipolar device such as an IGBT as below.
For example, in the conventional SiC bipolar device, a stacking fault occurs from a basal plane dislocation in the active region and is spread by collisions of the minority carrier injected in the active region under the forward biasing. As a result, as the SiC bipolar device stays active, the degradations in the forward direction such as the increase of the ON-resistance and the decrease of the current density occur. The SiC-IGBT, one type of the bipolar device, also suffers from the degradation in the forward direction such as the decrease of collector-emitter current density Jce. As the impurity concentration in the nCEL becomes higher, more basal plane dislocations are propagated into the nCEL and remain and thus the degradation in the forward direction becomes larger.
Further, when an IGBT is switched from the OFF state to the ON state by applying the gate voltage under the forward biasing, holes (minority carrier) injected from the p+ collector layer 1 enter from the location in the nCEL 75 near the corner 82 of the p body region 76 to the p body region 76, and flows to the emitter electrode 72 across the p body region 76. When an IGBT is switched from the ON state to the OFF state by reducing the gate voltage under the forward biasing, holes (minority carrier) flowing in the portion in the nCEL 75 near the corner 82 of the p body region 76 disappear at the end of all.
Therefore, the conventional SiC-IGBT has a problem that the stacking fault in the nCEL 75 near the corner 82 of the p body region 76 becomes apparent since holes are concentrated near the corner 82 when the switch is turned on or off under the forward biasing. As a result, the forward direction characteristics in the nCEL 75 near the corner 82 degrade significantly.
Thus, even if the conventional SiC-IGBT increase the collector-emitter current density Jce with the nCEL 75, the stacking fault due to the basal plane dislocations left in the nCEL 75 increases the ON voltage and reduces the collector-emitter current density Jce, thereby degrading the forward direction characteristics. As the degradation of the forward direction characteristics prevails, it becomes possible that the device is destroyed by large current that flows when the switch is turned on.
Metal atoms tend to gather in the stacking fault and the metal atom condensed stacking fault can be a path of leak current. When metal atoms gather at a stacking fault that is present across the junction between the p body region 76 and the nCEL 75, leak current flowing through the stacking fault reduces the breakdown voltage. A stacking fault present at the junction near the corner 82 of the p body region 76 where electric field most concentrates under the forward biasing significantly reduces the breakdown voltage, leading to the breakdown of a device in some cases. It is found for the first time that such a problem manifest itself as the operation frequency of an IGBT gets higher.